1. Field of the Invention
The present invention relates to an air-fuel ratio control for internal combustion engine which feedback controls an air-fuel ratio of air-fuel mixture to a target air-fuel ratio based on a deviation of a current output of an air-fuel ratio sensor.
2. Description of Related Art
Conventionally, there has been implemented an air-fuel ratio feedback control apparatus for carrying out feedback control of the air-fuel ratio based on the output of an oxygen sensor (O.sub.2 sensor) which generates a stepwise voltage signal in dependence on whether the air-fuel ratio is on the rich side or the lean side of the stoichiometric air-fuel ratio. In an air-fuel ratio control apparatus employing an oxygen sensor, feedback control of the air-fuel ratio is started when the oxygen sensor starts outputting a voltage signal. With the exhaust gas regulation enforced in recent years, on the other hand, another air-fuel ratio sensor such as a linear A/F sensor of a limit current type which is capable of outputting a current signal linearly is used to feedback control of the air-fuel ratio in place of the oxygen sensor (JP-A 8-201334). This linear air-fuel ratio sensor outputs a current representing the air-fuel ratio in response to a voltage applied to the sensor. In the case of the linear air-fuel ratio sensor, a feedback gain of the feedback control can be set at a higher level.
Such a linear air-fuel ratio sensor starts outputting the current even before reaching a temperature of activation. Before attaining the temperature of activation, however, the output current representing the actual air-fuel ratio becomes smaller than a current which is to be output after the temperature of activation is reached or the sensor response characteristics becomes slow. If the feedback control of the air-fuel ratio is started while the air-fuel ratio sensor is still before the activation, the feedback control will be carried out with the correlation of the current output of the sensor to the actual air-fuel ratio remaining shifted. This will degrade drivability. In addition, when the feedback control is started later due the fact that the air-fuel ratio sensor is in a cold state (in an inactive state), the air-fuel ratio control to the highest purification range of a catalyst is delayed. It is thus desirable to start the feedback control at an earlier stage.